1. Field of the Invention
The present invention relates to an accelerating self-focusing cavity for charged particles, intended more especially to form the first or single accelerating cavity of an accelerating structure of a linear accelerator.
2. Description of the Prior Art
In linear accelerators for charged particles, it is known that an important difficulty is due to defocusing of the charged particle beam along the accelerating structure and more particularly in the first part of the accelerator; this effect asserting itself all the more so since the energy of the particles is still low.
One of the consequences of the defocusing of the beam is that it increases the number of particles situated outside of the acceptance range of the accelerators; this causes degradation of the qualities of the accelerated beam of particles obtained, such for example as the energy homogeneity, and may also affect the over-all efficiency of the accelerator.
Thus, correction of this defocusing effect is particularly advantageous in the first or possibly only accelerating cavity of the accelerating structure. This effect may be corrected by adding solenoids, disposed concentrically about the accelerating structure, so as to create a corrective magnetic field.
Despite its cost, this solution may be comtemplated if the structure has a small section, the electro-magnetic wave injected into this structure for accelerating the beam having a high frequency (generally 3000 MHz); the wave length is then small and the resonance frequency of the accelerating cavities is obtained with small geometrical dimensions of these latter.
This solution is all the more expensive and difficult to apply when the section of the accelerating structure increases and becomes practically impossible to apply when the linear accelerator operates with metric waves; the diameter of the accelerating structure then being between one and two meters.